Fluids Available
Hydrochloric acid
Preflush
Overflush
Hydrofluoric acid systems
Mud Acid
Organic Mud Acid
Clay Acid
Organic Acids
Formic
Acetic
Citric (L1)
Selection Criteria
Formation mineralogy
Sensitivity
Deconsolidation
Precipitation
Fines release
Reactivity
Chemical composition
Surface area
Rock Structure
HCl solubility / carbonates
Clay distribution
Preflush - Brine
Ammonium chloride
Minimum spacing at any moment between formation brine and HCl
3 wt% (<5% Clay)
4 wt% (5-10%)
5 wt% (10-15%)
6 wt% (>15%)
Main Acid - Volume
1 Target: skin reduction
2 Never zero
3 Minimum skin achievable: pseudoskin value
4 Maximum reduction: 90%
Systems Available
Mud Acid
Organic Mud Acid
Clay Acid
Organic Clay Acid
Mud Acid
1 Nine HCI-HF formulations in MA selection Guide
2 Dissolves siliceous minerals
3 Dowell offered the first commercial Mud Acid Service in 1940 in the (U.S. Gulf Coast)
Mud Acid Reaction Simplified Version
Organic Mud Acid
1 Formic acid (9% L036 replaces 12% HCl)
2 Less corrosive than comparable Mud Acid formulations
3 Reaction rate ~ 1/4 that of Mud Acid
4 Reduces sludged tendency
Organic Mud Acid Preparation
Dissolving Y-1 (Ammonium Bifluoride) in HCl solution
Add enough HCl to completely react with Y-1
Add L36 last
Blending 20% HF (H200) and L36 into fresh water
Clay Acid:
a retarded Mud Acid
a retarded Mud Acid
Improved Penetration with Clay Acid
Clay Acid & Mud Acid
1 Preflush to Mud Acid: sensitivity
2 Main Acid: carbonate-cemented, long MA treatments
3 Overflush to Mud Acid: enhanced clay control
4 Shut-in and bring production back slowly
Primary Reaction – What to do ?
Alkali compounds
1. Push away formation brine
2. Brine with no Na, K, Ca
Ammonium Chloride Brine
CaF2
Dissolve CaCO3
HCl Preflush
Secondary Reaction – What to do ?
AlFx
Maintain a low-pH environment
Silica Gel
Maintain a low-pH environment
Include HCl in HF treatment (Mud Acid)
Tertiary Reaction – What to do?
AlFy
1. Maintain a low-pH environment
pH > 2.5 a problem
May be localized in the formation
2. Overdisplace fluids
HCl overflush
Matrix Stimulation: Carbonate
Kinetics of HCl Reaction
Mass-Transfer-Limited
Key Factors in Carbonate Acidizing
1. Penetration
2. Acid reactivity
3. Injection rates
4. Diversion
Injection Rates: dissolution patterns
Patterns change depending on:
Temperature
Injection velocity
Surface reaction rate
Impact of Pump Rate and Temperature
Reaction rate can be too high even with Organic Acids at high temperature.
Core entrance after 15% HCl flow
Wormhole Pattern from Radial Flow
Pore Level Model